Field of the Invention
The invention relates to a method for locating a jamming source jamming signals of a satellite navigation system and an associated system.
Description of Related Art
The term “jamming source” is understood to mean a source of intentional jamming or a source of interference.
A satellite navigation system, or GNSS for global navigation satellite system, uses a constellation of satellites moving around the earth in very precisely determined orbits. Thus, it is possible to know, at any instant, the position of any satellite. The orbits of satellites are chosen so that at any time, six to 12 satellites are visible from any point on the earth. Each satellite transmits multiple radio signals of determined modulation and frequency. On the ground or on a land, sea or air vehicle, a receiver receives the signals transmitted by visible satellites.
An on-board satellite navigation system receiver measures the duration of propagation required for a timestamp transmitted by a satellite to reach it. The timestamps are encoded in carrier waves via the phase modulation technique. Each satellite thus transmits a set of pseudo-random codes that are specific thereto. A replica of the code sequence is generated by the receiver and the shift that the replica must undergo in order to coincide with the received code corresponds to the propagation time required by the signal to travel the satellite—receiver distance. This time multiplied by the speed of light in the traversed medium gives a distance measurement referred to as the pseudo-distance. Based on measurements of the pseudo-distances separating it from each visible satellite, and on knowledge of the position of the satellites, the receiver deduces its precise position in terms of latitude, longitude and altitude in a terrestrial frame of reference through numerical resolution akin to triangulation. Based on phase (Doppler) measurements of the carriers, and on precise knowledge of the apparent speed of the satellites, the receiver precisely calculates the speed. It may also deduce therefrom the precise date and time in the temporal frame of reference of the satellite navigation system.
The reception of the satellite signals and the precision of the measurements remains very sensitive, despite broadening spreading codes and increasing transmission powers, to the presence of jamming and interference sources.
The services making fundamental use of GNSS location and time transfer systems are many and play an important role in the economy and functioning of state infrastructure.
As such, protecting GNSS services from jamming sources is one of the imperatives of states, which must deploy detection and location means over the entire territory so as to ensure that the signal reception environment is in accordance with usage standards.
The proposed invention falls within the scope of a extended system for monitoring the quality of the signal reception environment, including all GNSS constellations, which system is dedicated to the detection and location of sources of interference or jamming liable to degrade the quality of position, speed and time measurements delivered by GNSS systems.
The invention relates in particular to the capability of using rail, air and maritime—and even road—transport platforms to relax the grid of a fixed surveillance network, and concentrate the surveillance around transport routes of goods and persons.
Faced with the first demonstrations of the vulnerabilities of receivers and the presence of low-cost personal jammers, conventional means for analysing the spectrum of GNSS signals are gradually being deployed in proximity to critical infrastructure such as airports, ports, communication centres, nuclear power plants, etc.
However, such means (covering the national territory with a network of spectral analysis stations allowing strong jammers to be detected) are relatively ineffective in the case of distant wideband jammers, as the level of jamming sufficient to interfere with GNSS signals is much lower than the standard level of thermal noise.
This makes it very difficult to detect weak jamming sources, which are nonetheless capable of interfering with the operation of receivers, and to predict their impacts on the operation of user receivers located in the vicinity of the analyser antenna.
Such solutions require a large number of stations in order to cover the territory, hence a high cost. Furthermore, the detection area is limited to around each station. The location of a strong jamming source is done by crossing the levels measured over multiple stations.